If you're like many Prevention readers, multivitamins have been a key part of your daily routine since... well, forever. As recently as 2002, no less an authority than the Journal of the American Medical Association recommended that "all adults take one multivitamin daily." We at Prevention have suggested them to you dozens of times over the years as well. And many doctors and nutritionists still urge a multivitamin to any "less-than-perfect eater" to compensate for dietary shortfalls.

But today, a tsunami of scientific data has resulted in a reversal in thinking among many experts in the health and nutrition community, including Miriam Nelson, PhD, director of the John Hancock Research Center on Physical Activity, Nutrition, and Obesity at Tufts University. "The multivitamin as insurance policy is an old wives' tale, and we need to debunk it," she says.

The sea change is supported by two massive studies. The first, a review of 63 randomized, controlled trials (the gold standard research method) on multivitamins, published by the Agency for Healthcare Research and Quality, found that multis did nothing to prevent cancer or heart disease in most populations (the exception being developing countries where nutritional deficiencies are widespread). In the second paper, published last year, scientists at the Fred Hutchinson Cancer Research Center followed 160,000 postmenopausal women for about 10 years. The researchers' conclusion: "Multivitamins failed to prevent cancer, heart disease, and all causes of death for all women. Whether the women were healthy eaters or ate very few fruits and vegetables, the results were the same," says the lead author, Marian Neuhouser, PhD.

Maybe you never expected your multi to prevent breast cancer or head off a heart attack. Maybe you just felt that taking one would make you healthier by boosting your immunity or energy level. But research on those benefits is equally discouraging, especially in specialized groups on which you'd expect them to have an impact. For instance, a British review of eight studies found no evidence that multis reduced infections in older adults. Another study found that the vitamins didn't improve fatigue among breast cancer patients undergoing radiation therapy. And inner-city schoolchildren who took a multi did not perform any better on tests or have fewer sick days than students who didn't take one.

"There is even a small body of evidence that may suggest harm from a multi," says David Katz, MD, MPH, director of the Prevention Research Center at Yale University School of Medicine. A 2010 study of Swedish women found that those who took multivitamins were 19% more likely to be diagnosed with breast cancer over a 10-year period than those who didn't. A 2007 paper in the Journal of the National Cancer Institute found that men who took multivitamins along with other supplements were at increased risk of prostate cancer. And other research has linked excessive folic acid intake to higher colon cancer risk in people who are predisposed. "In terms of a risk-benefit ratio," says Dr. Katz, "why would you accept even a tiny risk if you're not getting any benefit?"

Buyer beware

It still shocks people that the FDA doesn't regulate supplements, including vitamins, minerals, and herbs, the same way it does for drugs. Supplements don't have to go through any safety testing before they hit stores. Despite 2007 legislation that marginally increased the FDA's authority, health and safety critics say the FDA doesn't have enough resources to oversee the industry.

Some new bills proposed in Congress would give the FDA more oversight and enforcement muscle, but in the meantime, these aisles are truly buyer beware zones.

Protect yourself: Choose products with seals from quality-testing companies like United States Pharmacopeia (USP) or NSF. Supplement companies pay to have their products evaluated according to these more rigorous protocols, which check for contaminants and ensure products contain exactly what their labels say.

You can also subscribe to ConsumerLab.com, an independent testing organization, for reports on specific supplement groups, like omega-3s. Finally, don't take any supplements without talking with your doctor. Many interact with medications or can worsen health conditions.

So why were earlier researchers so wrong? One reason is that they were studying the wrong people. It's now well known that people who take vitamins tend to be some of the planet's healthiest to begin with. Researchers have shown that vitamin takers tend to be leaner, more affluent, and more educated. They drink and smoke less; they exercise and go to the doctor more. In other words, they're healthy despite their use of multis.

In addition, the very concept of a multivitamin as nutrient delivery system is limited. We now have a much better understanding of how well whole foods deliver their nutritional benefits. A typical multi contains 10 to 25 isolated nutrients, but fruits and vegetables have hundreds of active compounds with a long list of health properties. "The vitamin C in a multivitamin is likely just not as effective as the vitamin C in a citrus fruit, where it's also surrounded by fiber and flavonoids and carotenoids. All these nutrients working together is what really keeps you healthy," explains Dr. Neuhouser.

So even when you're not eating the healthiest diet, there's no proof that a multivitamin is the right tool to fill in the gaps. "And it doesn't make up for the main disease-fighting nutrients the average American woman is missing, like fiber, omega-3s, and vitamin D," says Dr. Katz.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

For his part, Dr. Katz no longer recommends multis to most of his patients, nor does he take them himself. Neither does Kathleen Fairfield, MD, associate chief of medicine at Maine Medical Center and coauthor of the 2002 JAMA article that recommended multivitamins as a prudent health measure. Many Prevention advisors who specialize in cancer or nutrition research recently told us they've stopped taking and recommending multis as well.

So should you quit cold turkey? It's true that the average American woman—whether a healthy eater or not—probably won't benefit from a multivitamin. But the supplements are still recommended for certain groups, based on eating habits or lifestyle. And most women do still need a few strategic supplements to make up for certain dietary gaps.

To help with your decision, Prevention interviewed more than 10 nutritionists, doctors, and supplement experts and examined USDA, FDA, and other government data. The chart below reveals how their advice applies to you.

If you are... And your daily diet includes these foods... Do you still need a multivitamin? ... Prevention recommends these supplements

Yes. During pregnancy, requirements increase slightly for a number of nutrients, including vitamin C, folic acid and other B vitamins, and iron. Choose a prenatal formula with at least 600 mcg folic acid and 27 mg iron, or ask your doctor to prescribe one.